Bluetooth: HFP: Divide hfp_internal.h

Divide hfp_internal.h into two parts, HFP HF part hfp_hf_internal.h and
HFP common part hfp_internal.h.

Includes "hfp_internal.h" in file "hfp_ag_internal.h".
Includes "hfp_ag_internal.h" in file "hfp_ag.c".

Includes "hfp_internal.h" in file "hfp_hf_internal.h".
Includes "hfp_hf_internal.h" in file "hfp_hf.c".

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>